Process for filling etched holes using photoimageable thermoplastic polymer

ABSTRACT

A process for filling one or more etched holes defined in a frontside surface of a wafer substrate. The process includes the steps of: (i) depositing a layer of a photoimageable thermoplastic polymer onto the frontside surface and into each hole; (ii) reflowing the polymer; (iii) selectively removing the polymer from regions outside a periphery of each hole, the selective removing comprising exposure and development of the polymer; (iv) optionally repeating steps (i) to (iii) until each hole is overfilled with the polymer; and (v) planarizing the frontside surface to provide one or more holes filled with a plug of the polymer. Each plug has a respective upper surface coplanar with the frontside surface.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of application Ser. No. 15/046,239,entitled PROCESS FOR FILLING ETCHED HOLES, filed on Feb. 17, 2016, whichclaims priority under 35 U.S.C. §119(e) to U.S. Provisional PatentApplication Ser. No. 62/117,385, entitled PROCESS FOR FILLING ETCHEDHOLES, filed on Feb. 17, 2015, the content of which is incorporated byreference herein in its entirety for all purposes.

FIELD OF THE INVENTION

This invention relates to a process for filling etched holes. It hasbeen developed primarily to improve the planarity of filled holes inorder to facilitate subsequent MEMS fabrication steps.

BACKGROUND OF THE INVENTION

The Applicant has developed a range of Memjet® inkjet printers asdescribed in, for example, WO2011/143700, WO2011/143699 andWO2009/089567, the contents of which are herein incorporated byreference. Memjet® printers employ a stationary pagewidth printhead incombination with a feed mechanism which feeds print media past theprinthead in a single pass. Memjet® printers therefore provide muchhigher printing speeds than conventional scanning inkjet printers.

In order to minimize the amount of silicon, and therefore the cost ofpagewidth printheads, each Memjet® printhead IC is fabricated via anintegrated CMOS/MEMS process to provide a high nozzle packing density. Atypical Memjet® printhead IC contains 6,400 nozzle devices, whichtranslates to 70,400 nozzle devices in an A4 printhead containing 11Memjet® printhead ICs.

As described in U.S. Pat. No. 7,246,886, the contents of which areincorporated herein by reference, a typical printhead fabricationprocess for Memjet® printhead ICs requires etching of holes in afrontside of a CMOS wafer via DRIE (deep reactive ion etching), fillingthe holes with a sacrificial material (e.g. photoresist) to provide aplanar frontside surface, and then subsequently building MEMS nozzledevices on the frontside of the wafer. After completion of the allfrontside MEMS fabrication steps, the wafer is thinned from the backsideand trenches are etched from the backside to meet with the filledfrontside holes. Finally, all sacrificial material is removed fromfrontside holes and MEMS nozzle chambers by oxidative ashing. In theresulting printhead IC, the frontside holes define individual inletchannels for nozzle chambers.

A critical stage of fabrication is plugging the frontside holes withsacrificial material and planarizing the frontside surface of the wafer.If the frontside surface is not fully planar, then any lack of planarityis carried through subsequent MEMS fabrication steps and, ultimately,may lead to defective devices or weakened MEMS structures with shorterinstalled lifetimes.

One process for plugging holes formed by DRIE is described in U.S. Pat.No. 7,923,379. In this prior art process, a hole is filled in multiplestages by spinning on sequential layers of a photoresist. After each ofthese stages, the photoresist on the front surface of the wafer isselectively exposed and developed to leave only photoresist partiallyfilling the hole. The remaining photoresist inside the hole is hardbakedand the process repeated until the hole is fully filled withphotoresist. The aim is to provide a hole plugged with photoresist atthe end of the process, whereby an upper surface of the photoresist plugis coplanar with a frontside surface of wafer. This is the idealfoundation for subsequent MEMS fabrication steps on the frontsidesurface of the wafer.

However, the process described in U.S. Pat. No. 7,923,379 has a numberof drawbacks. Firstly, it is not possible to achieve true planarity atthe end of the process, because the hole is usually slightly overfilledor underfilled after the final exposure and development steps. Secondly,photoresist is highly viscous, which inhibits the escape of solvent orair bubbles. Bubbles can escape from the relatively thin final layer ofphotoresist, but cannot readily escape from the layer(s) of photoresistat the bottom of the hole. During thermal curing, these trapped solventbubbles may combine and expand to form relatively large voids, withconsequent instability in the plug. Thirdly, photoresists typicallycontract during thermal curing (hardbaking′). Contraction of thephotoresist during hardbaking also affects the stability of the plug.Thus, even if a planar upper surface can be achieved, the photoresistplug may be susceptible to ‘dishing’ during subsequent MEMS fabricationssteps; and any lack of stability in the photoresist plug may lead toproblems in subsequently constructed MEMS structures e.g. nozzle platecracking.

Thermoplastic polymers, which typically have lower viscosities than mostphotoresists and can be reflowed when heated, offer a potential solutionto at least some of the problems associated with trapped solvent bubblesand contraction of photoresist as described above. However,thermoplastic polymers are not usually photoimageable and requireplanarizing via a chemical-mechanical planarization (CMP) process.Although a CMP process is technically possible for thermoplasticpolymers, it is not practically feasible for thick layers of polymer,which are required to fill relatively deep holes formed by DRIE. This isdue to: (1) poor stopping selectivity on the frontside surface whenplanarizing thick layers of polymer; (2) the rate of CMP beingunacceptably slow for large scale fabrication; (3) rapid ‘gumming’ ofCMP polishing pads, which consequently require regular replacement.

It would be desirable to provide an alternative process for fillingphotoresist holes, which ameliorates at least some of the problemsdescribed above.

SUMMARY OF THE INVENTION

In a first aspect, there is provided a process for filling one or moreetched holes defined in a frontside surface of a wafer substrate, saidprocess comprising the steps of:

(i) depositing a layer of a thermoplastic first polymer onto thefrontside surface and into each hole;

(ii) reflowing the first polymer;

(iii) exposing the wafer substrate to a controlled oxidative plasma soas to reveal the frontside surface;

(iv) optionally repeating steps (i) to (iii);

(v) depositing a layer of a photoimageable second polymer so as tooverfill each hole with said second polymer;

(vi) selectively removing the second polymer from regions outside aperiphery of the holes to provide overfilled holes, the selectiveremoving comprising exposure and development of the second polymer; and

(vii) planarizing the frontside surface to provide one or more holesfilled with a plug comprising the first and second polymers, each plughaving a respective upper surface coplanar with the frontside surface,wherein the first and second polymers are different.

The process according to the first aspect advantageously provides arobust process for plugging high aspect ratio holes formed by DRIE. Inparticular, the process provides a plug which is substantially free ofbubbles by virtue of using a relatively low viscosity first polymerhaving thermoplastic reflow properties, which allows bubbles to readilyescape during deposition and reflow. Further, the process provides astable foundation for subsequent MEMS processes by virtue of employing areflowable thermoplastic first polymer, which uniformly fills thefrontside hole. Still further, the process provides a frontside plughaving an upper surface coplanar with the frontside surface by virtue ofplanarizing step (typically chemical-mechanical planarizing).Planarization (e.g. by CMP) is facilitated by use of the photoimageablesecond polymer for the final filling step, which is removed from regionsoutside the periphery of each hole by conventional exposure anddevelopment. Thus, a minimal amount of the second polymer needs to beremoved by planarization, which enables high throughput, good stoppingselectivity and minimal gumming of CMP polishing pads (i.e. lowerconsumable costs). These and other advantages will be apparent to theperson skilled in the art from the detailed description of the firstembodiment below.

Preferably, the first polymer is less viscous than the second polymer.As foreshadowed above, a relatively low viscosity first polymerfacilitates escape of trapped solvent and air bubbles, resulting in amore robust plug.

Preferably, each hole has a depth of at least 5 microns or at least 10microns. Typically, each hole has depth in the range of 5 to 100 micronsor 10 to 50 microns.

Preferably, each hole has an aspect ratio of >1:1. Typically, the aspectratio is in the range of 1.5-5:1

In one embodiment, steps (i) to (iii) may be repeated one or more times.In other embodiments, steps (i) to (iii) may be performed only once. Inan alternative embodiment, steps (i) and (ii) may be repeated one ormore times, and step (iii) may be performed only once.

Preferably, an extent of overfill of the hole immediately prior to step(vi) is less than about 12 microns or less than about 10 microns.Minimal overfill is desirable to facilitate subsequent planarization.

Typically, additional MEMS fabrication steps are performed on theplanarized frontside surface of the wafer substrate. In a preferredembodiment, the additional MEMS fabrication steps construct inkjetnozzle devices on the planarized frontside surface of the wafersubstrate. Each nozzle device may comprise a nozzle chamber in fluidcommunication with at least one hole, and a respective inlet for eachnozzle chamber may be defined by one of said holes.

Preferably, the additional MEMS fabrication steps include at least oneof: wafer thinning and backside etching of ink supply channels. Each inksupply channel preferably meets with one or more filled holes to providefluid connections between the backside and frontside of the wafer. Eachink supply channel is usually relatively wider than the frontside holes.

A final stage of MEMS fabrication preferably employs oxidative removal(“ashing”) of the first and second polymers from the holes. Oxidativeremoval typically employs an oxygen-based plasma, as known in the art.

In a second aspect, there is provided a process for filling one or moreetched holes defined in a frontside surface of a wafer substrate, saidprocess comprising the steps of:

(i) depositing a layer of a photoimageable thermoplastic third polymeronto the frontside surface and into each hole;

(ii) reflowing the third polymer;

(iii) selectively removing the third polymer from regions outside aperiphery of each hole, the selective removing comprising exposure anddevelopment of the third polymer;

(iv) optionally repeating steps (i) to (iii) until each hole isoverfilled with the third polymer; and

(v) planarizing the frontside surface to provide one or more holesfilled with a plug of the third polymer, each plug having a respectiveupper surface coplanar with the frontside surface.

The process according to the second aspect makes use of a special classof thermoplastic photoimageable polymers. The desirable property ofthermoplasticity enables the third polymer to be reflowed so as to enjoythe same advantages as those described above in connection with thefirst polymer. Furthermore, the desirable property of photoimageabilityenables the third polymer to be removed from regions outside a peripheryof the holes by conventional photolithographic exposure and development.Accordingly, the process according to the second aspect obviatesoxidative removal of the first polymer (as described above in connectionwith the first aspect), whilst still enjoying the advantages of: ahighly robust plug; coplanarity of the plug and frontside surfacefollowing planarizing; and efficient planarization by virtue ofphotolithographic removal of the majority of the third polymer prior toplanarization.

Preferably, the process according to the second aspect comprises only asingle sequence of steps (i) to (iii), wherein each hole is overfilledwith the third polymer after step (iii).

Other preferred embodiments, where relevant, which are described abovein connection with the first aspect are of course applicable to thesecond aspect.

In a third aspect, there is provided a process for filling one or moreetched holes defined in a frontside surface of a wafer substrate, theprocess comprising the steps of:

(i) depositing a layer of a thermoplastic first polymer onto thefrontside surface and into each hole;

(ii) reflowing the first polymer;

(iii) optionally repeating steps (i) and (ii) until the holes areoverfilled with the first polymer;

(iv) depositing a layer of a photoimageable second polymer;

(vi) selectively removing the second polymer from regions outside aperiphery of the holes, the selective removing comprising exposure anddevelopment of the second polymer;

(vii) exposing the wafer substrate to a controlled oxidative plasma soas to reveal the frontside surface of the wafer substrate; and

(viii) planarizing the frontside surface to provide one or more holesfilled with a plug comprising the first polymer only, each plug having arespective upper surface coplanar with the frontside surface,

wherein the first and second polymers are different.

The process according to the third aspect is analogous in many respectsto the process according to the first aspect. However, in the thirdaspect, the second polymer is used merely to provide a relativelythicker polymeric layer over each hole, each hole being initiallyoverfilled with the first polymer. Therefore, the oxidative removal stepensures that a cap of polymeric material remains over each hole prior toplanarization. This is advantageous because any solvent or air bubblesin the second polymer, which may be present at the interface between thefirst and second polymers, are removed during the planarization step.Hence, the plug of material filling the hole is solely the thermoplasticfirst polymer, which provides a very robust foundation for subsequentMEMS fabrication steps.

In some embodiments, the process may comprise the additional step of:exposing the wafer substrate to a controlled oxidative plasma so as toreveal the frontside surface of the wafer substrate after step (ii).

Other preferred embodiments, where relevant, which are described abovein connection with the first aspect are of course applicable to thethird aspect.

In a fourth aspect, there is provided a process for filling one or moreetched holes defined in a frontside surface of a wafer substrate, saidprocess comprising the steps of:

(i) depositing a layer of a photoimageable fourth polymer onto thefrontside surface and into each hole;

(ii) selectively removing the fourth polymer from regions outside aperiphery of each hole, the selective removing comprising exposure anddevelopment of the fourth polymer;

(v) optionally repeating steps (i) and (ii) until each hole isoverfilled with the fourth polymer; and

(vi) planarizing the frontside surface to provide one or more holesfilled with a plug of the fourth polymer, each plug having a respectiveupper surface coplanar with the frontside surface.

The process according to the fourth aspect is most suitable for fillingrelatively shallower (i.e. less than 10 microns) or low aspect ratio(i.e. less than 1:1) holes. The fourth polymer is typically conventionalphotoresist, which is not thermoplastic and cannot, therefore, bereflowed. Nevertheless, efficient planarization is still achievablesince the amount of fourth polymer to be removed by CMP is minimized.

Other preferred embodiments, where relevant, which are described abovein connection with the first aspect are of course applicable to thethird aspect.

As used herein, the term “hole” generally means any cavity, via ortrench defined in a wafer substrate. By definition, each hole has afloor and sidewalls extending upwards therefrom to meet with a surfaceof the wafer substrate. Each hole may have any shape in cross-section,such as circular, oblong, rounded oblong, square, rounded square, oval,elliptical etc. Likewise, the hole may be in the form of an elongatetrench. In the present context, elongate trenches may be used as ‘dicingstreets’ for dicing silicon wafers into individual chips.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the present invention will now be described by way ofexample only with reference to the accompanying drawings, in which:

FIG. 1 is a schematic side view of a silicon substrate having a highaspect ratio hole etched in frontside surface;

FIG. 2 shows the substrate shown in FIG. 1 after deposition of athermoplastic first polymer;

FIG. 3 shows the substrate shown in FIG. 2 after reflowing and curing ofthe first polymer;

FIG. 4 shows the substrate shown in FIG. 3 after oxidative removal ofthe first polymer from the frontside surface;

FIG. 5 shows the substrate shown in FIG. 4 after deposition of aphotoimageable second polymer;

FIG. 6 shows the substrate shown in FIG. 5 after exposure anddevelopment of the second polymer;

FIG. 7 shows the substrate shown in FIG. 6 after chemical-mechanicalplanarization;

FIG. 8 shows the substrate shown in FIG. 1 after deposition of athermoplastic photoimageable third polymer;

FIG. 9 shows the substrate shown in FIG. 8 after reflowing and curing ofthe third polymer;

FIG. 10 shows the substrate shown in FIG. 9 after exposure anddevelopment of the third polymer;

FIG. 11 shows the substrate shown in FIG. 10 after chemical-mechanicalplanarization;

FIG. 12 shows the substrate shown in FIG. 1 after repeated depositionand reflow baking of the thermoplastic first polymer;

FIG. 13 shows the substrate shown in FIG. 12 after deposition of thephotoimageable second polymer;

FIG. 14 shows the substrate shown in FIG. 13 after exposure anddevelopment of the second polymer;

FIG. 15 shows the substrate shown in FIG. 14 after oxidative removal ofthe first polymer from the frontside surface;

FIG. 16 shows the substrate shown in FIG. 15 after chemical-mechanicalplanarization;

FIG. 17 is a schematic side view of a silicon substrate having a lowaspect ratio hole etched in frontside surface;

FIG. 18 shows the substrate shown in FIG. 17 after deposition of aconventional photoimageable polymer;

FIG. 19 shows the substrate shown in FIG. 18 after exposure anddevelopment;

FIG. 20 shows the substrate shown in FIG. 10 after chemical-mechanicalplanarization;

FIG. 21 is a perspective view of inkjet nozzle devices each having achamber inlet defined in a frontside surface of a silicon substrate; and

FIG. 22 is a sectional side view of the inkjet nozzle device shown inFIG. 21.

DETAILED DESCRIPTION OF THE INVENTION First Embodiment

Referring to FIG. 1, there is shown a substrate 1 having a high aspectratio hole 2 defined in a frontside surface 3 thereof. The substrate isa CMOS silicon wafer having an upper CMOS layer 5 disposed on a bulksilicon substrate 4. The CMOS layer 4 typically comprises one more metallayers interposed between interlayer dielectric (ILD) layers. The hole 2may be defined by any suitable anisotropic DRIE process (e.g. ‘Boschetch’ as described in U.S. Pat. No. 5,501,893). The hole 2 may have anydesired shape in cross-section, the shape being defined by a photoresistmask during the etching process.

FIG. 2 shows the substrate 1 after spin-coating a reflowablethermoplastic polymer 7 onto the frontside surface 3 followed bysoft-baking. The thermoplastic polymer 7 is non-photoimageable and maybe of any suitable type known to those skilled in the art. For example,the thermoplastic polymer 7 may be an adhesive, such as a polyimideadhesive. A specific example of a suitable thermoplastic polymer 7 isHD-3007 Adhesive, available from HD MicroSystems™.

Soft-baking after deposition of the thermoplastic polymer 7 removessolvent to provide a tack-free film. Since the thermoplastic polymer 7has a relatively low viscosity (e.g. <1500 Cps), any air or solventbubbles present in the polymer can readily escape during soft-baking.Still referring to FIG. 2, it can be seen that the thermoplastic polymer7 is readily deposited inside the high aspect ratio hole 2 duringspin-coating due to it relatively low viscosity.

Referring now to FIG. 3, there is shown the substrate 1 afterreflow-baking at a relatively higher temperature than soft-baking. Thisreflow-baking step raises the thermoplastic polymer 7 to a temperatureabove its glass transition temperature, allowing the polymer to reflowand fill the hole 2 more completely. For example, reflow-baking may beperformed at about 300° C., while soft-baking may be performed at about90° C.

Depending on the depth and aspect ratio of the hole 2, as well as thetype of thermoplastic polymer 7 employed, the steps described inconnection with FIGS. 2 and 3 may be repeated one or more times untilthe hole is filled to a level just below the frontside surface, as shownin FIG. 3. The hole 2 may be >60% filled, >70% filled, >80% or >90%after all spin-coating and reflowing steps have been completed.

After the hole 2 has been partially-filled to a desired level, thethermoplastic polymer 7 is then cured at a relatively higher temperaturethan the reflow baking temperature in order to cross-link and harden thepolymer. The resultant plug of thermoplastic polymer 7 shown in FIG. 3is substantially free of any air or solvent bubbles. Moreover, thereflow step(s) ensure the thermoplastic polymer 7 uniformly contactssidewalls of the hole 2 to provide a robust foundation for subsequentMEMS processing.

Turning now to FIG. 4, the substrate 1 is shown after removal of apredetermined thickness of the thermoplastic polymer 7 via a controlledoxidative removal process (“ashing”). Typically, the controlledoxidative removal process comprises a timed exposure to an oxygen-basedplasma in a conventional ashing oven. A planar thickness of polymerremoved by the ashing process is proportional to the period of ashing.As shown in FIG. 4, the ashing process removes a thickness of thethermoplastic polymer 7, such that removal is complete from thefrontside surface 3 in regions outside the periphery of the hole 2.However, the hole 2 remains partially-filled with the thermoplasticpolymer 7 by virtue of the additional thickness of polymer in the hole.

Next, as shown in FIG. 5, a conventional photoimageable(non-thermoplastic) polymer 9 is deposited onto the frontside surface 3of the substrate 1 by spin-coating followed by soft-baking. Thephotoimageable polymer 9 is spin-coated to a thickness of about 8microns so as to overfill the hole 2. The photoimageable polymer 9 maybe of any suitable type known to those skilled in the art. For example,the photoimageable polymer 9 may be a polyimide or a conventionalphotoresist. A specific example of a suitable photoimageable polymer 9is HD-8820 Aqueous Positive Polyimide, available from HD MicroSystems™.

Referring to the FIG. 6, the photoimageable polymer 9 is then exposedand developed, by conventional methods known to those skilled in theart, so as to remove substantially all of the polymer 9 from regionsoutside a periphery of the hole 2. The resultant substrate 1 has anoverfilled hole 2 having an 8 micron “cap” of the photoimageable polymer9.

Following final curing of the photoimageable polymer 9, the frontsidesurface 3 of the substrate 1 is then subjected to chemical-mechanicalplanarization (CMP) so as to remove the cap of photoimageable polymer 9and provide a planar frontside surface, as shown in FIG. 7.Advantageously, the amount of photoimageable polymer 9 that is requiredto be removed by CMP is relatively small due to the previous exposureand development steps described in connection with FIG. 6. Hence, theCMP process has acceptable process times (e.g. 5 minutes or less), goodstopping selectivity and minimal gumming of CMP pads, which reduces thecost of consumables.

In the resultant substrate 1, shown in FIG. 7, the hole 2 is pluggedwith the thermoplastic polymer 7 and the photoimageable polymer 9. Thispolymer plug is robust and substantially free of any solvent or airbubbles. Furthermore, an upper surface 11 of the plug is coplanar withthe frontside surface 3 by virtue of the final planarizing process. Theplugged hole therefore provides an ideal foundation for subsequentfrontside MEMS processing steps, such as fabrication of inkjet nozzlestructures.

Second Embodiment

A second embodiment of the present invention will now be described withreference to FIGS. 8 to 11. Referring firstly to FIG. 8 the hole 2 isfilled with a polymer 13 having both thermoplastic and photoimageableproperties. An example of the thermoplastic photoimageable polymer 13 isLevel® M10 coating, available from Brewer Science. The thermoplasticphotoimageable polymer 13 has a relatively low viscosity which iscomparable to the thermoplastic polymer 7 described hereinabove. Thepolymer 13 is therefore able to fill the hole 2 in a single spin-coatingfollowed by soft-baking to removal solvent. The low viscosity andthermoplastic reflow properties of the polymer 13 enable any solvent orair bubbles to escape during soft-baking and reflow baking.

FIG. 9 shows the polymer 13 after reflow-baking at a relatively highertemperature than soft-baking. This reflow-baking step raises the polymer13 to a temperature above its glass transition temperature, allowing thepolymer to reflow and ensure the hole 2 is overfilled.

Referring to the FIG. 10, the thermoplastic photoimageable polymer 13 isthen exposed and developed by conventional methods known to thoseskilled in the art, so as to remove substantially all of the polymer 13from regions outside a periphery of the hole 2. The resultant substrate1 has an overfilled hole 2 with a “cap” of the polymer 13.

Following final curing (e.g. UV curing) of the thermoplasticphotoimageable polymer 13, the frontside surface 3 of the substrate 1 isthen subjected to chemical-mechanical planarization (CMP) so as toremove the cap of polymer 13 and provide a planar frontside surface, asshown in FIG. 11. Advantageously, the amount of polymer 13 that isrequired to be removed by CMP is relatively small due to the previousexposure and development steps described in connection with FIG. 10.Hence, the CMP process has acceptable process times (e.g. 5 minutes orless), good stopping selectivity and minimal gumming of CMP pads, whichreduces the cost of consumables.

In the resultant substrate 1, shown in FIG. 11, the hole 2 is pluggedwith the thermoplastic photoimageable polymer 13. This polymer plug isrobust and substantially free of any solvent or air bubbles.Furthermore, an upper surface 15 of the plug is coplanar with thefrontside surface 3 by virtue of the final planarizing process. Theplugged hole therefore provides an ideal foundation for subsequentfrontside MEMS processing steps, such as fabrication of inkjet nozzlestructures.

Third Embodiment

Referring to FIGS. 12 to 16, there is shown a third embodiment of thepresent invention employing the first polymer 7 and the second polymer9, as described above in connection with the first embodiment. FIG. 12shows the substrate 1 after spin-coating of the thermoplastic firstpolymer 7 and reflow baking. By contrast with the first embodiment, thehole 2 is overfilled with the polymer 7, typically using two or morecycles of spin-coating and reflow baking. After reflow baking, thesubstrate 1 may be exposed to an oxidative plasma to remove the polymer7 from the frontside surface 3. However, this step is optional and FIG.12 shows an alternative process where there is no ashing step after eachcycle of spin-coating and reflow baking.

Referring to FIG. 13, the photoimageable second polymer 9 is thenspin-coated on the substrate 1 over the thermoplastic polymer 7.Subsequent masked exposure and development of the second polymer 9removes the second polymer from regions outside a periphery of the hole2. Accordingly, as shown in FIG. 14, a relatively thick polymeric layer,comprised of the first polymer and second polymer 9, is disposed overthe hole 2; and a relatively thin polymeric layer, comprised of thefirst polymer 7, is disposed over the remainder of the frontside surface3 in regions outside a periphery of the hole 2.

Referring to FIG. 15, the substrate 1 is then exposed to a controlledoxidative plasma (“ashing”) so as to remove a predetermined thickness ofpolymeric material. The first polymer 7 is removed completely fromregions outside a periphery of the hole 2 to reveal the frontsidesurface 3. However, since a relatively thick polymeric layer wasdisposed over the hole 2 prior to ashing, a polymeric cap 17 remainsover the hole after the ashing step, as shown in FIG. 15.

Finally, as shown in FIG. 16, the frontside surface is subjected tochemical-mechanical planarization (CMP) to remove the polymeric cap 17,stopping on the frontside surface 3. The process according to the thirdembodiment advantageously provides a plug of the first polymer 7 fillingthe hole 2. Moreover, an upper surface 19 of the plug of the firstpolymer 7 is coplanar with the frontside surface 3.

The process according to the third embodiment is potentiallyadvantageous compared to the first embodiment by avoiding any of thesecond polymer 9 in the final plugged hole. Therefore, any solvent orair bubbles present in the second polymer 9, which may grow at aninterface between the first and second polymers, are avoided in thefinal plugged hole.

Fourth Embodiment

The fourth embodiment described herein is suitable for fillingrelatively shallow and/or low aspect ratio holes (e.g. holes having anaspect ratio of <1:1 and/or holes have a depth of less than 10 micronsor less than 5 microns). FIG. 17 shows the silicon substrate 1 having alow aspect ratio hole 21 defined in a frontside surface 3 thereof.

FIG. 18 shows the substrate 1 after spin-coating a conventionalphotoimageable polymer 23 onto the frontside surface 3 followed bysoft-baking. The photoimageable polymer 23 may be of any suitable typeknown to those skilled in the art, such as polyimide or photoresist.

The hole 17 is intentionally overfilled with the polymer 23 and then thepolymer is subsequently removed from regions outside the periphery ofthe hole by conventional exposure and development steps. FIG. 19 showsthe substrate 1 after exposure and development of the polymer 23; thehole 17 is plugged with the polymer and has a cap of polymeric materialprotruding from the frontside surface 3.

Following final curing of the photoimageable polymer 23, the frontsidesurface 3 of the substrate 1 is then subjected to chemical-mechanicalplanarization (CMP) so as to remove the cap of polymer 23 and provide aplanar frontside surface, as shown in FIG. 20. Advantageously, theamount of polymer 23 that is required to be removed by CMP is relativelysmall due to the previous exposure and development steps described inconnection with FIG. 19. Hence, the CMP process has acceptable processtimes (e.g. 5 minutes or less), good stopping selectivity and minimalgumming of CMP pads, which reduces the cost of consumables.

Moreover, the plug of polymer 23 has a uniform upper surface 25, whichis coplanar with the frontside surface 3. The plugged hole thereforeprovides a good foundation for subsequent frontside MEMS processingsteps.

Although the process described above in connection with the fourthembodiment employs a single hole-filling step, it will be appreciated bythose skilled in the art that the hole may be filled in multiple stages,similar to the process described in U.S. Pat. No. 7,923,379. However, incontrast with the process described in U.S. Pat. No. 7,923,379, theprocess according to the third embodiment overfills the hole forsubsequent planarization (see FIGS. 18 and 19).

MEMS Inkjet Nozzle Devices

By way of completeness, there will now be described an inkjet nozzledevice fabricated by leveraging the hole-filling process describedabove.

Referring to FIGS. 21 and 22, there is shown an inkjet nozzle device 10comprising a main chamber 12 having a floor 14, a roof 16 and aperimeter wall 18 extending between the floor and the roof. FIG. 21shows a CMOS layer 20, which may comprise a plurality of metal layersinterspersed with interlayer dielectric (ILD) layers.

In FIG. 21 the roof 16 is shown as a transparent layer so as to revealdetails of each nozzle device 10. Typically, the roof 16 is comprised ofa material, such as silicon dioxide or silicon nitride.

The main chamber 12 of the nozzle device 10 comprises a firing chamber22 and an antechamber 24. The firing chamber 22 comprises a nozzleaperture 26 defined in the roof 16 and an actuator in the form of aresistive heater element 28 bonded to the floor 14. The antechamber 24comprises a main chamber inlet 30 (“floor inlet 30”) defined in thefloor 14. The main chamber inlet 30 meets and partially overlaps with anendwall 18B of the antechamber 24. This arrangement optimizes thecapillarity of the antechamber 24, thereby encouraging priming andoptimizing chamber refill rates.

A baffle plate 32 partitions the main chamber 12 to define the firingchamber 22 and the antechamber 24. The baffle plate 32 extends betweenthe floor 14 and the roof 16.

The antechamber 24 fluidically communicates with the firing chamber 22via a pair of firing chamber entrances 34 which flank the baffle plate32 on either side thereof. Each firing chamber entrance 34 is defined bya gap extending between a respective side edge of the baffle plate 32and the perimeter wall 18.

The nozzle aperture 26 is elongate and takes the form of an ellipsehaving a major axis aligned with a central longitudinal axis of theheater element.

The heater element 28 is connected at each end thereof to respectiveelectrodes 36 exposed through the floor 14 of the main chamber 12 by oneor more vias 37. Typically, the electrodes 36 are defined by an uppermetal layer of the CMOS layer 20. The heater element 28 may be comprisedof, for example, titanium-aluminium alloy, titanium aluminium nitrideetc. In one embodiment, the heater 28 may be coated with one or moreprotective layers, as known in the art.

The vias 37 may be filled with any suitable conductive material (e.g.copper, tungsten etc.) to provide electrical connection between theheater element 28 and the electrodes 36. A suitable process for formingelectrode connections from the heater element 28 to the electrodes 36 isdescribed in U.S. Pat. No. 8,453,329, the contents of which areincorporated herein by reference.

Part of each electrode 36 may be positioned directly beneath an end wall18A and baffle plate 32 respectively. This arrangement advantageouslyimproves the overall symmetry of the device 10, as well as minimizingthe risk of the heater element 28 delaminating from the floor 14.

As shown most clearly in FIG. 21, the main chamber 12 is defined in ablanket layer of material 40 deposited onto the floor 14 and etched by asuitable etching process (e.g. plasma etching, wet etching etc.). Thebaffle plate 32 and the perimeter wall 18 are defined simultaneously bythis etching process, which simplifies the overall MEMS fabricationprocess. Hence, the baffle plate 32 and perimeter wall 18 are comprisedof the same material, which may be any suitable etchable ceramic orpolymer material suitable for use in printheads. Typically, the materialis silicon dioxide or silicon nitride.

A printhead 100 may be comprised of a plurality of inkjet nozzle devices10. The partial cutaway view of the printhead 100 in FIG. 21 shows onlytwo inkjet nozzle devices 10 for clarity. The printhead 100 is definedby a silicon substrate 102 having the passivated CMOS layer 20 and aMEMS layer containing the inkjet nozzle devices 10. As shown in FIG. 21,each main chamber inlet 30 meets with an ink supply channel 104 definedin a backside of the printhead 100. The ink supply channel 104 isgenerally much wider than the main chamber inlets 30 and provides a bulksupply of ink for hydrating each main chamber 12 in fluid communicationtherewith. Each ink supply channel 104 extends parallel with one or morerows of nozzle devices 10 disposed at a frontside of the printhead 100.Typically, each ink supply channel 104 supplies ink to a pair of nozzlerows (only one row shown in FIG. 21 for clarity), in accordance with thearrangement shown in FIG. 21B of U.S. Pat. No. 7,441,865.

The printhead 100 may be fabricated by building the MEMS layercontaining inkjet nozzle devices 10 on a wafer substrate having theplugged hole shown in FIG. 7. The planarized frontside surface 3 of thesubstrate facilitates frontside MEMS fabrication processes. Afterfrontside MEMS fabrication steps are completed, the wafer is thinnedfrom a backside and the ink supply channels 104 are etched from thebackside to meet with the plugged frontside holes. Finally, the polymerplug (e.g. polymers 7 and 9) is removed from the frontside hole 2 byoxidative ashing to define the main chamber inlets 30.

It will, of course, be appreciated that the present invention has beendescribed by way of example only and that modifications of detail may bemade within the scope of the invention, which is defined in theaccompanying claims.

1. A process for filling one or more etched holes defined in a frontsidesurface of a wafer substrate, said process comprising the steps of: (i)depositing a layer of a photoimageable thermoplastic polymer onto thefrontside surface and into each hole; (ii) reflowing the polymer; (iii)selectively removing the polymer from regions outside a periphery ofeach hole, the selective removing comprising exposure and development ofthe polymer; (iv) optionally repeating steps (i) to (iii) until eachhole is overfilled with the polymer; and (v) planarizing the frontsidesurface to provide one or more holes filled with a plug of the polymer,each plug having a respective upper surface coplanar with the frontsidesurface.
 2. The process of claim 1, wherein each hole has a depth of atleast 10 microns.
 3. The process of claim 1, wherein each hole has anaspect ratio of >1:1.
 4. The process of claim 1, wherein steps (i) to(iii) are not repeated and each hole is overfilled with the polymerafter step (iii).
 5. The process of claim 1, wherein, in step (v), thewafer is planarized by a chemical-mechanical planarization (CMP)process.
 6. The process of claim 1, wherein an extent of overfill of thehole immediately prior to step (v) is less than 12 microns.
 7. Theprocess of claim 1 further comprising additional MEMS fabrication steps.8. The process of claim 7, wherein the additional MEMS fabrication stepsconstruct inkjet nozzle devices on the planarized surface of the wafersubstrate.
 9. The process of claim 8, wherein each nozzle devicecomprises a nozzle chamber in fluid communication with at least onehole.
 10. The process of claim 9, wherein a respective inlet for eachnozzle chamber is defined by one of said holes.
 11. The process of claim10, further comprising at least one of: wafer thinning and backsideetching of ink supply channels.
 12. The process of claim 11, whereineach ink supply channel meets with one or more filled holes.
 13. Theprocess of claim 12, wherein each ink supply channel is relatively widersaid one or more holes.
 14. The process of claim 13, further comprisingoxidative removal of the first and second polymers from the holes.